In phase change memory cells, a limiting factor preventing minimization of the dimensions of the phase change volume is the breakdown voltage. For example, in a memory cell configuration with the phase change volume along a generally vertical current path between top and bottom electrodes, a voltage drop in the phase change volume occurs vertically in the resistive phase change volume. If the voltage drop occurs across a phase change volume that is too thin, then the electric field is too high, and breakdown occurs in the phase change volume. Accordingly, the phase change volume has a minimum thickness to prevent such voltage breakdown. Such a minimum thickness also addresses a read disturb issue. Accordingly, present phase change memory cells must be configured with a phase change material thickness in excess of an amount actually required to display phase change behavior.
In a phase change memory cell configuration, the active region volume (which changes phases) of the phase change material increases with the size of the adjacent heater or contact. Because the active region which undergoes the phase change is a volume with a thickness larger than the maximum width of the adjacent heater or contact, the resulting mushroom-like appearance also lends its name to this phase change memory cell configuration.
In the presentation “Current Status of Phase Change Memory and its Future” by Stefan Lai of Intel Corp. at IEDM 2003, the phase change element has a thickness which exceeds the width of the bottom electrode, such that the phase change element is large enough to support the hemispherical volume of the active region.